1. Field of the Invention
The present invention relates to a solar cell and, more particularly, to a solar cell which makes use of a ferroelectric material or ferroelectric materials.
2. Description of the Related Art
Generally, a solar cell is provided with a structure where pairs of electrons and holes are generated in a semiconductor by way of the light from the outside, and electric fields are formed by way of a pn junction so that the electrons move to the n type semiconductor, and the holes move to the p type semiconductor, thereby producing the required electric power.
In a buried contact solar cell (BCSC) that bears a high efficient solar cell structure, a groove is formed at the front surface of the solar cell, and a conductive material fills the internal space of the groove to thereby form a front metallic electrode of a buried type. In the BCSC structure, an SiO2-based layer is deposited onto the entire front surface of the cell except for the groove area to obtain a surface passivation effect and an anti-reflection effect.
In addition to SiO2, an anti-reflection layer may be deposited with TiO2, MgF2, ZnS and SiNx and so on, while realizing the desired surface passivation effect through ion implantation, plasma hydrogenated treatment, SiNx deposition by plasma enhanced chemical vapor deposition (PECVD).
Furthermore, in the BCSC structure, an Al-based layer is deposited, and heat-treated to thereby form a rear electrode with a heavily doped region. Consequently, the open-circuit voltage is increased by way of a rear surface field (BSF) effect, thereby enhancing the efficiency of the solar cell.
However, with such a structure, damage is done to the cell structure during the process of heat-treating the Al-based layer for the rear electrode so that recombination of the electrons and the holes occurs increasingly at the surface area. In order to solve such a problem, a solar cell with a double side buried contact (DSBC) structure where the rear electrode is also buried in the form of a groove is introduced.
However, in the DSBC structure, a shunt path is made between the rear electrode and a floating junction layer so that the desired BSF effect cannot be obtained.
U.S. Pat. No. 6,081,017 discloses an improved technique of solving the above problems. A self-biased solar cell structure is introduced to reduce the degree of recombination of the electrons and the holes at the rear surface area while increasing the open-circuit voltage of the cell and enhancing the energy efficiency thereof. In the structure, a dielectric layer is deposited onto the entire surface of the substrate except for the rear electrode area, and a layer for a voltage application electrode is deposited thereon. The voltage application electrode is connected to the front electrode for the solar cell to thereby produce the desired self-biased voltage. The self-biased voltage is applied to the rear surface area while forming the desired rear surface field there. In this way, the loss of carrier recombination is reduced while enhancing the energy efficiency of the solar cell.
However, in such a technique, the process of connecting the front electrode to the rear electrode should be additionally conducted, and this results in complicated processing steps. As the voltage from the solar cell is used for obtaining the desired rear surface field, the dimension of the rear surface field is limited to the value less than the open-circuit voltage.
U.S. Pat. No. 4,365,106 discloses a solar cell using a ferroelectric material. In the solar cell, the conversion of the optical energy to the electrical energy is made by way of the variation in polarization as a function of the temperature of the ferroelectric material. In the metal-insulator-semiconductor (MIS) structure, a ferroelectric material is used as an insulating material. The temperature of the ferroelectric material may be altered due to the light from the outside. In this case, the surface polarization of the ferroelectric material is varied while generating electric charge. The electric charge induces a strong electric field between the ferroelectric material and the semiconductor to thereby form an inversion layer. The inversion layer severs to make the desired pn junction. Therefore, the pairs of electrons and holes generated due to the light from the outside are separated from each other by way of the internal electric field to thereby produce the desired electrical energy as with the usual solar cell.
However, in such a technique, hetero-junction is made at the pn junction interface between the ferroelectric material and the semiconductor so that the loss by the interfacial recombination of the electrons and the holes is increased. Furthermore, the electrons do not move about well due to the insulating effect of the ferroelectric material, and this lowers the efficiency of the solar cell.
It is, therefore, an object of the present invention to provide a high efficiency solar cell which involves simplified structural components as well as simplified processing steps.
It is another object of the present invention to provide a high efficiency solar cell which makes use of a ferroelectric material or ferroelectric materials.
These and other objects may be achieved by a solar cell where the front surface or the rear surface thereof or the front and rear surfaces thereof are formed with a ferroelectric layer.
Specifically, the solar cell has a pn structure with a semiconductor substrate of a first conductive type, a semiconductor layer of a second conductive type formed on the first conductive type semiconductor substrate, and a pn junction formed at the interface between the first conductive type semiconductor substrate and the second conductive type semiconductor layer. The first and the second conductive types are opposite to each other in polarity. A front electrode is placed over the pn structure while being connected to the second conductive type semiconductor layer. A rear electrode is placed below the pn structure while being connected to the first conductive type semiconductor substrate. A ferroelectric layer is formed on one of the front surface of the second conductive type semiconductor layer and the rear surface of the first conductive type semiconductor substrate. A poling electrode is formed on at least a part of the ferroelectric layer.
The ferroelectric layer is formed with a ferroelectric material selected from BaTiO3, BST((Ba,Sr)TiO3), PZT((Pb,Zr)TiO3), or SBT(SrBi2Ta2O7).
The poling electrode placed on the ferroelectric layer at the front surface of the second conductive type semiconductor layer is formed with a transparent conductive oxide material selected from ITO (indium tin oxide), RuO2, SrRuO3, IrO2, or La1-xSrxCoO3.
The poling electrode placed on the ferroelectric layer at the rear surface of the semiconductor substrate is formed with a metallic electrode material selected from Al, Cu, Ag, or Pt.